1. Field of the Invention
The present invention relates to a pump electrode of a gas sensor such as a NOx sensor, and a conductive paste used to form the same.
2. Description of the Related Art
Conventionally, various kinds of measurement devices are used to know a concentration of a desired gas component in a measurement gas. For example, as a device to measure a NOx concentration in a measurement gas such as a combustion gas, a NOx sensor formed of an oxygen ion conducting solid electrolyte such as zirconia (ZrO2) is well known (refer to Japanese Patent Application Laid-Open No. 2006-284223 and U.S. Pat. No. 3,537,983, for example).
In a NOx sensor using current-limiting method as disclosed in the Japanese Patent Application Laid-Open No. 2006-284223 or the U.S. Pat. No. 3,537,983, a NOx concentration in a measurement gas is determined based on the fact that there is a linear relation between a current value of a current (NOx current) flowing when an oxygen ion generated due to reduction or decomposition of NOx in a measurement electrode is pumped by a measurement pump cell serving as an electrochemical pump cell, and the NOx concentration.
Here, in the NOx sensor using the current-limiting method, the introduced measurement gas needs to be supplied to the measurement electrode after the gas is put into a sufficiently low oxygen partial pressure state, in order to ensure measurement accuracy of the NOx concentration. When the NOx sensor is what is called a serial two-chamber structure type disclosed in the Japanese Patent Application Laid-Open No. 2006-284223 or the U.S. Pat. No. 3,537,983, first, the measurement gas is introduced into a first chamber (referred to as an inner space also) and its oxygen concentration is adjusted to be approximately constant by a pumping operation (oxygen is pumped in or out) of a main pump cell serving as an electrochemical pump cell, and then introduced into a second chamber. In the second chamber, the oxygen is pumped out by a pumping operation of an auxiliary pump cell serving as an electrochemical pump cell similarly, so that the measurement gas is put into a sufficiently low oxygen partial pressure state.
Meanwhile, as for the NOx sensor having the above configuration, if NOx is reduced or decomposed at a certain level or more in the oxygen pumping cell such as the main pump cell or the auxiliary pump cell, the accurate NOx concentration cannot be obtained, as a matter of course. Also, when another oxide gas is decomposed, the control of the oxygen concentration in the chamber is substantially disturbed, so that the measurement accuracy deteriorates. Therefore, in general, a main pump electrode constituting the main pump cell in the chamber and an auxiliary pump electrode constituting the auxiliary pump cell in the chamber are formed of a material whose reducing ability with respect to a NOx component in the measurement gas is weakened. For example, a noble metal powder including Pt as a main component and added with Au, or an alloy powder of Pt and Au is preferably used as one example (refer to Japanese Patent Application Laid-Open No. 11-64272 and Japanese Patent Application Laid-Open No. 2001-66289, for example).
In order to improve the measurement accuracy in the current-limiting type NOx sensor to obtain the NOx concentration based on the above-described principle, it is necessary to reduce the reduction or decomposition of NOx and sufficiently lower the oxygen partial pressure (that is, oxygen concentration) of the measurement gas which is supplied to the measurement electrode to the extent not to affect the measurement of NOx. Therefore, it is necessary to configure the oxygen pumping cell such that the oxygen gas is selectively decomposed and pumped.
In addition, in the process of studying to implement the accurate control of the oxygen concentration, the inventor of the present invention has found the fact that there is a variation in oxygen pumping ability of the main pump cell and the auxiliary pump cell in the NOx sensor even when they are the same lots produced under the same production condition, and the fact that the variation in oxygen pumping ability appears as a variation in inter electrode impedance of the main pump electrode and the auxiliary pump electrode, in the NOx sensor produced by the conventional method. Since the variation in lot-to-lot oxygen pump ability prevents a NOx sensor quality from being stabilized, it has to be eliminated.